1. Field of the Invention
The present invention relates to a direct type of tire air pressure detection system which directly attaches transmitters provided with pressure sensor to wheels to which tires are attached, has the transmitters send out detection signals from the pressure sensors, and has a receiver attached to the chassis side receive them so as to detect the tire air pressure.
2. Description of the Related Art
In the past, one type of tire air pressure detection system has been the direct type. In this type of tire air pressure detection system, the wheel side where tires are attached has transmitters provided with pressure sensors directly attached to them. The chassis side has antennas and a receiver provided at it. When detection signals from the pressure sensors are sent from the transmitters, the detection signals are received by the receiver through the antennas and the tire air pressure is detected (for example, see Japanese Unexamined Patent Publication (Kokai) No. 14-257661).
Summarizing the problem to be solved by the invention, when this direct type of tire air pressure detection system is used in an environment subjected to a strong electric field or in an environment where there is strong radio interference, the strong electric field or strong radio interference will affect the detection signals of the sensors provided at the transmitters and cause the detection signals to become mistaken in some cases. In such cases, data containing the mistaken detection signals will be sent from the transmitters resulting in poor detection accuracy of tire air pressure. For example, despite a tire air pressure being normal, data indicating an abnormality will be sent from a transmitter, whereby the receiver side will mistakenly recognize that the tire air pressure in question is abnormal and issue a mistaken warning that the tire air pressure has fallen.
To prevent such a mistaken warning, it may be considered to employ a shield structure for protecting the sensors from a strong electric field or strong radio interference, however there are limits to such a shield effect and mistaken warning would not therefore always be able to be prevented.